In the industrial fabrication of hollow glass containers using known methods such as press-and-blow and blow-blow, it is frequently found that the thickness of glass varies locally in the wall of the container, even for articles that are simple in shape, such as cylindrical bottles. The outside surface that comes into contact with the mold during forming generally has the desired shape. As a result, variations in distribution of glass and thus in wall thickness are manifested by deformation of the inside surface. When such deformation is small, it is of no consequence on the strength or on the appearance of containers. In contrast, poor distribution of material can lead to defective appearance that can sometimes be troublesome, and even worse, it can lead to certain locations where there is no glass. It is considered that a good distribution of glass is a distribution that is of uniform thickness, such that the inside and outside surfaces are substantially parallel throughout. In conventional methods, a poor distribution of glass is characterized and detected by measuring glass thickness, where such measurements are generally point or localized measurements.
In the prior art, various solutions have been proposed for detecting light refracting defects. For example, patent FR 2 794 241 proposes a machine that is adapted to detect refraction defects without turning the container.
That machine includes a conveyor designed to bring containers for inspection up to an inspection station. The inspection station has a camera situated on one side of the conveyor and adapted to take an image of the container. The inspection station also has a light source situated on the other side of the conveyor and associated with means for defining light intensity that varies continuously in cyclic manner in space between dark and bright extremes for the light source, with a rate of change that is less than that needed for detecting defects. Refractive defects of the container have a lens effect, which means that they present to the camera portions of the light source in compressed form. Such a compressed image of the light source with intensity variation at a rate that is greatly increased improves detection of a refractive defect by increasing its contrast.
In practice, that technique is not capable of detecting light refracting defects that have little refracting power.
Another known technique, e.g. as described in U.S. Pat. No. 5,004,909, proposes a device for inspecting the walls of a container, the device comprising a camera for observing a light pattern through a container that is being driven in rotation, which light pattern is made up of alternating white and black stripes. The image of the container is analyzed while taking account of the deformations of the white and black strips due to the presence of a light refracting defect, that can thus be detected.
In practice, that technique is found to be highly sensitive to the distribution of the material constituting the containers. If the glass is distributed in a manner that is not uniform but still acceptable, then the refractions caused by the slope of the inside surface have the effect of greatly deforming the patterns, such that it becomes practically impossible to recognize them, measure them, and analyze them in the images. Consequently, for such productions, it does not appear to be possible to distinguish refracting defects of the containers from irregularities in the wall thickness of the containers. It appears to be possible to detect refracting defects only on productions in which irregularities in the wall thickness of containers are frequent and acceptable.
A method and a device are also known from patent application FR 2 907 553 comprising a light source that is controlled to produce a first type of lighting that is uniform and a second type of lighting that is made up of alternating dark zones and bright zones with discontinuous spatial variation. That device also has means for taking images of articles illuminated by the first and second types of lighting in order to detect respectively high contrast defects and low contrast defects.
Although such a device enables both types of defect to be detected using a single source, that device finds it difficult to detect certain types of refractive defect, mainly because only one image is taken with the second type of lighting. Specifically, high contrast defects are detected with the uniform light source. Additionally, low contrast defects are detected with the single image that is obtained when the source presents alternating black and white stripes with sharp edges and thus with variation that is discontinuous. Deformations of the sharp edges of the striped pattern are analyzed in the image together with local contrast produced by refractive defects, as is done in U.S. Pat. No. 5,004,909. When the container possesses variations in thickness, and thus a poor distribution of glass, the deformations of the pattern are considerable and do not enable low contrast defects to be detected effectively.
Patent application FR 2 958 040 describes a method and an installation for detecting the presence and the extent of defects in an optical component, which method consists in producing a periodic light pattern that is transmitted through the optical component held stationary in an inspection station, in acquiring successive images in transmission through the component of the periodic pattern, which pattern is phase-shifted for each acquisition, in calculating phase images from those successive images, and in analyzing said phase images in order to deduce therefrom the presence of defects.
In practice, that technique is not suitable for in-line inspection of transparent or translucent containers traveling at a high rate between a light source and a system, since it requires the inspected articles to be stopped for a considerable length of time in order to acquire a plurality of images.